ancestral stem form more closely than does any one of the other three
American species of weasels. If this judgment is correct, the shape of
the tympanic bullae of the American weasels may be explained as
follows: In the subspecies of _Mustela frenata_ of the temperate
regions of North America the bullae have most nearly been pushed out of
the braincase and at the same time have undergone some enlargement. The
subspecies of this same species in Central and South America represent
an earlier stage in the evolution of American weasels and retain less
inflated bullae--less inflated even than those of the southern
subspecies of _erminea_. _M. africana_ probably separated from the stem
form at a still earlier time if we may judge by the lesser inflation of
its tympanic bullae. There are other reasons for thinking that
_africana_ separated from the stem form earlier than _M. frenata_ did.
During the time that elapsed since the separation of _M. frenata_ from
the stem form, the tympanic bullae of _M. erminea_ probably increased
slightly in size, as probably also did the brain but without shoving
the auditory complex forward from its former position.
DISTRIBUTION AND SPECIATION
Weasels of the subgenus _Mustela_ are known from the Pleistocene but
not from deposits laid down at an earlier time (see page 10). The
Pleistocene weasels from Rancho La Brea of southern California and from
Potter Creek Cave and Samwel Cave, both of northern California, are
subspecifically indistinguishable from the weasels living in those same
localities today. The other notable occurrence of weasels in the
Pleistocene is in the Conard Fissure of Arkansas. Brown (1908:181, 182,
pl. 17) names two kinds from the Fissure. One is an extinct subspecies
(_Mustela frenata gracilis_) possibly of the species which occurs in
the same region today and the other, _Mustela erminea? angustidens_,
is an extinct subspecies of a species which occurs only farther north
today. _M. erminea_ came south, probably in front of one of the ice
sheets, as did several other species of American mammals, now of more
northern distribution, that left their remains in Conard Fissure.
_Mustela rixosa_ is not recorded as a fossil in America although it is
known from the "Diluvial" deposits of the Old World; see Woldrich
(1884:1000), who employs the name "_Foetorius minutus_ n. sp.," and see
also Zimmerman (1943:295-296).
The ermine, _Mustela erminea_, is the most generalized of the full
species. For example, the number of teeth is as large as in any other
species and greater than in certain species. The teeth are
sharp-pointed, uncrowded, and individually less specialized than in any
other American weasel. M1 has the inner half, or lobe, of approximately
the same size as the outer lobe instead of much larger than the outer
lobe (the outer lobe is the larger in several other species). The
tympanic bullae are less inflated and less protruded from the
braincase. The skull is rounded, and has no marked crests and ridges
whereas the skulls of the other species are more pronouncedly modeled
and sculptured. Therefore, it is possible to think of these other
species as derived from _M. erminea_. A derivation in the reverse
direction would be more difficult. From the foot soles of an ermine, or
a weasel closely resembling an ermine, the more complex soles of
_Mustela africana_ could have been derived by a decrease in hairiness,
although it would be necessary to suppose that the thenar pad has been
retained in _africana_ and has been lost in the living _erminea_. The
alternate possibility, namely, that the thenar pad was a relatively
recent acquisition in the _africana_ line seems less probable. The tail
of _erminea_ is of "average" length and in size of entire animal
_erminea_ is intermediate between the other American weasels.
Structurally, _Mustela erminea_ appears to be nearest the stem form
from which all of the living weasels ascended. Its present holarctic
distribution is in harmony with the view that it is a direct descendant
from the stem form because the stem forms of most of the known kinds of
mustelids appear to have lived in the holarctic region. To be sure,
_Mustela erminea_ is regarded as having undergone some progressive
change in structure, but less than the other weasels, in the period of
time when the weasels were evolving from the stem form.
The least weasel, _Mustela rixosa_, seems to be an ancient type and to
judge from the size and proportions of its parts, was differentiated
from the _erminea_ stem at a time earlier than were the other American
Recent species of weasels. In size, in reduction of the tail, and in
proportions of the skull, _M. rixosa_ is, in each instance, the most
aberrant of all the weasels, _Mustela nivalis_ of Europe and western
Asia included. This aberrancy results from the retention of certain
primitive features, in the teeth and basicranial region, and from
specialization in proportions of the skull. The skull is long, deep,
and narrow. These proportions probably are adaptations permitting the
animal to follow the smaller kinds of mice into their burrows. In most
of that part of North America where _erminea_ and _rixosa_ occur
together, _erminea_ is a much larger animal and takes as prey almost
all kinds of land vertebrates that it is powerful enough to kill. These
include varying hares and ptarmigans. The least weasel, _rixosa_, can
hardly manage such large prey and lives on the smaller rodents.
_Mustela rixosa_ may eat numbers of insects (see page 176 beyond),--a
kind of food which _Mustela erminea_ is not known to eat. Apparently
the two species are able to live in the same areas because each eats a
somewhat different kind of food than does the other and hence they do
not compete to the point where one is crowded out by the other. This is
the case in the latitudes where the two species of weasels are of
different bodily size, but in the southernmost latitudes where these
two species occur, _erminea_ becomes almost as small as _rixosa_ and
only one of the species, to the exclusion of the other, occurs in a
given area. All through the Rocky Mountains, south of Montana and in
the territory west of these mountains all the way to the Pacific Coast,
only the small subspecies of _erminea_ is to be found. In the
Alleghenies of the eastern United States only _rixosa_ occurs. In New
England where _erminea_ approaches the size of _rixosa_, the latter is
unknown. Probably this exclusiveness results from competition for food,
although competition for dens, safe breeding places and other
requirements of life may be involved.
The species _erminea_ invaded the western United States and in the
process of invasion probably developed there the small size appropriate
to permit _erminea_ to live in that latitude before it could do the
same thing in the Appalachian region. Later than _erminea_, the least
weasel, _Mustela rixosa_, which was small to begin with, also spread
southward from the holarctic region, stopped short in the western
United States at the northern boundary of the area in which _erminea_
was of small size, but in the Appalachian region of the eastern United
States continued on southward to the limits of temperature tolerant for
it because _erminea_ had not yet penetrated into that region and no
other small carnivore was there to offer competition.
The long-tailed weasel, _Mustela frenata_, occurs mostly south of the
regions inhabited by the ermine, and mostly south of the region
inhabited by the least weasel which appears to live as well with
_frenata_ as with _erminea_. It is true that _erminea_ and _frenata_
occur in the same region, but this is a relatively narrow belt across
the United States; and from within it a person cannot go far either
north or south without reaching a region in which only one of the two
species occurs. Exception has to be made for the Rocky Mountains and
the Sierra Nevada, where _erminea_ is of exceptionally small size. In
these mountains and in the boreal mountainous parts of the intervening
region of the United States, _erminea_ and the large-sized _frenata_
occur together over a wide area. Presumably the two occupy different
ecologic niches, much as _rixosa_ and _frenata_ probably do where they
occur together.
Most of the geographic range of the long-tailed weasel, _M. frenata_,
is in the temperate region. Structurally, this species is the most
advanced of the American weasels. Its dentition is the most highly
specialized for cutting. M1 is relatively small and the inner lobe is
slightly larger than the outer lobe. The skull, throughout, is more
modeled than in the other species; the rostrum, the lower jaws and the
teeth--all parts of the offensive equipment--are well developed
relative to the corresponding structures in other weasels; the
basicranial region exhibits an advanced stage of development in that
the tympanic bullae show the maximum degree of inflation. Also, they
are thrust far out of the braincase, thereby providing more room for
the relatively larger brain which is protected by a more solidly built
braincase than in _erminea_.
Several subspecies of _Mustela frenata_ occur in the tropics, that is
to say, south of the Mexican tableland and on the coastal plain to the
east of it. Each is structurally more primitive than subspecies of the
temperate region. As compared with _Mustela frenata frenata_ of the
temperate Mexican tableland the size in these tropical subspecies is
smaller; the tail is shorter; the braincase and entire skull are less
modeled; the postorbital breadth is more; the teeth are smaller; the
deuterocone of P4 is not so far anterior to the protocone; the tympanic
bullae are less inflated, are farther removed from the foramen ovale,
and a larger proportion of each bulla is contained within the
braincase. These features serve to set off from northern races of
_frenata_ all those subspecies of _frenata_ which occur from southern
México southward to the northern and western limits of the Amazon
drainage of South America. The Amazon Basin is inhabited by another
species, _Mustela africana_, having more primitive characters.
In the species _frenata_, the explanation for this abrupt change in
characters between the animals of the temperate highlands and those of
the tropical lowlands may be this: In the early Pleistocene, after the
emergence of much or all of Central America took place, weasels
distributed themselves over the Isthmus and into South America. These
weasels were more generalized in structure than those now inhabiting
the uplands of México. Failure of this stock of weasels often to cross
some still-persisting water barrier, or failure of this stock to cross
some water barrier that was widened or reformed because of a rise in
sea level in some one of the interglacial periods of the Pleistocene
cut the _frenata_ stock into two or more parts. After the land
connection was established or re-established and when the necessary
precedent plants and rodents again had established themselves, the two
groups of weasels, one from the northern tableland of México, and the
other from the southern area of tropical complexion, met. The weasels
of the _frenata_ stock that reinvaded the area from the north probably
did so by following along the chain of high volcanic cones and narrow
uplifts. If and when a subsequent inundation occurred in some
part of Central America, weasels were stranded on the adjacent
mountains--converted into islands--only the higher parts of which were
above water. _Mustela frenata costaricensis_ and _Mustela frenata
goldmani_ may be examples of a northern stock of weasel that pushed
southward in the highlands and became stranded for a short time.
Following the latest emergence of land to provide a continuous highway
between the two continents, weasels from the south and the insular
populations, as for example, _M. f. costaricensis_, were the first to
invade the low tropical areas most recently under water. When the
Pleistocene history of Central America is better known, the facts will
provide a useful means of testing the hypothesis that has been outlined
immediately above.
As explained above, fossil specimens of _M. frenata_ from deposits of
the last half of Pleistocene time show that no appreciable change
occurred in some areas, for example, in the vicinity of Hawver Cave and
Samwel Cave of California, and that but slight change occurred in other
areas, for example, in southern California (fossils from Rancho La
Brea) and probably in the central United States (fossil from Conard
Fissure). It is possible to imagine, therefore, that the two groups of
weasels, one occurring southward only as far as the highlands of
Central America and the other occurring in northern South America, had
not differentiated sufficiently in the period of their isolation to
prevent crossbreeding when they last came into contact. If the
separation of the two groups had been maintained for a longer period,
the two groups, tropical weasels and austral weasels, probably would
have been so different when the two met as to prevent crossbreeding and
they would have constituted two full species instead of only one.
_Mustela africana_ is the most primitive of the American weasels. Some
of the most important structural features that mark it as such are in
the basicranial region. The tympanic bullae are less inflated than in
other weasels, are pointed anteriorly and posteriorly, and do not have
the lateral margins carried outward to the outer margins of the
braincase. The mastoid sinus is not involved, by inflation or marked
modification in the production of the auditory complex. Between the
alisphenoid and the squamosal there is a clear demarcation posteriorly
from a point directly lateral to the foramen ovale. This demarcation
permits a transverse rounding of the alisphenoid to form a longitudinal
ridge between the anterior margin of each bulla and the base of the
pterygoid of the same side. Nevertheless, there is no such
specialization of this primitive, structural feature such as occurs in
some African and Asiatic mustelids in which the tympano-pterygoid part
of the alisphenoid fuses with the tip of the hamulus of the pterygoid.
However, the tympano-pterygoid eminence has not been obliterated in _M.
africana_ as it has in the other American weasels. Another primitive
feature in the basicranial region of _M. africana_ is the tendency
toward separation of the paroccipital processes from the tympanic
bullae. The thenar pad of the foot probably is an inheritance from a
primitive ancestor since the pad is present in the viverrids and in a
majority of mustelids judged to be more primitive than _Mustela_.
Some specializations are obvious in _Mustela africana_. One is the
reduction in number of premolars; p2 is absent whereas it is normally
present in the other weasels; P2 has one instead of two roots; and, in
relation to the other teeth, m2 is smaller. The shortness of the
preorbital part of the skull in relation to the length of the skull as
a whole may reflect the mentioned reduction of the premolars or
retention of a primitive shape of skull, or both. Also, certain
features which denote immaturity in other weasels are retained in
adults of this species, as for example, sutures on the dorsal face of
the preorbital region of the skull.
[Illustration: FIGS. 17-22. Views of the feet of American weasels
(subgenus _Mustela_) to show differences in number and arrangement of
the pads and variation in degree of hairiness of the soles. × 1-1/2 In
each figure, left-forefoot on left, and left hind foot on right.
FIG. 17. _M. rixosa rixosa_, Halifax, N.S.; juv., [F], 7425 U.S.N.M.
Fig. 18. _M. erminea richardsonii_, Ft. Chimo; ad. [F], 14866 U.S.N.M.
Fig. 19. _M. frenata noveboracensis_, Mich., July 7, 1913; ad. [M],
44689 M.Z.
Fig. 20. _M. f. frenata_, Brownsville, June 1, 1892; yg. [M], 34043
U.S.N.M.
Fig. 21. _M. frenata panamensis_, Panamá, February 17, 1911; sad. [F],
type.
Fig. 22. _M. a. africana_, Pará, Brazil, Sept., 1908; yg. [M], 37475
A.M.N.H.
Figs. 17, 18 and 19. Drawn from specimens preserved in alcohol.
Figs. 20, 21 and 22. Drawn from relaxed feet of dried skins.]
_Mustela africana_, all characters considered, is the most aberrant of
the American weasels. That is to say, greater difference prevails
between _M. africana_ and any other American weasel than exists between
any other two American weasels. The distinctive cranial and dental
characters, excepting the reduction in number of premolars, are of a
primitive nature. For example, the relatively wide postorbital region,
the large braincase that is inflated anteriorly, and the flattened
tympanic bullae are points of resemblance to the holarctic _Mustela
erminea_, the species which is regarded as most closely resembling the
stem form. Also, the mentioned characters in adults of _M. africana_
resemble ontogenic stages passed through by other weasels.
Consequently, it is thought that _M. africana_ crossed the
filter-barrier from North America to South America, remained isolated
from the original stock for a length of time sufficient to permit
_africana_ to differentiate from North American weasels and _vice
versa_ to such a degree that crossbreeding with the _frenata_ stock was
prevented when _frenata_, at a later time, pushed southward over the,
then zoölogically less-effective, water barrier, or continental bridge
if it was by this time in existence.
[Illustration: FIG. 23. Diagram indicating probable relationships of
the species of American weasels.]
The four full species of American weasels may well be thought of as
having the same stem form of which _erminea_ is the most nearly direct
descendant. Geographic and climatic changes may have operated to
isolate, and then to foster morphologic differentiation of, first
_rixosa_ in Eurasia, next _africana_, third the _tropicalis_ section of
_M. frenata_, and finally _M. frenata_ itself, leaving _M. erminea_ as
a modern version, somewhat altered to be sure, of the stem form. Some
of these ideas are expressed in figure 16. The climate is different in
the ranges of the several species and the climate has changed through
time in the ranges of at least many subspecies. Natural selection of
morphological features best adapted to a particular kind of climate
probably has altered some species more than others. _M. erminea_ in
almost every one of its characteristics is generalized and potentially
progressive whereas _africana_ retains more characters which are truly
primitive along with a few which are specializations. _M. africana_ is
potentially the least progressive of any of the American weasels. The
most specialized weasels are the North American races of _Mustela
frenata_. A progressive series of increasing specialization is
comprised in (1) _M. africana_, (2) the _M. tropicalis_ (Central
American, lowland) section of _M. frenata_, and (3) the races of _M.
frenata_ in North America.
Considering now features of the environment which have obviously
influenced the distribution and speciation of weasels, water barriers
are important. Bering Strait, Carquinez Strait (along with San
Francisco Bay) which opens through the Golden Gate, and the channels
between the islands of southeastern Alaska, have contributed to the
formation of subspecies. The difference is really slight on the two
sides of Bering Strait and San Francisco Bay and is slightly more on
two sides of each of several of the channels between the islands of
southeastern Alaska. The differences between the weasels on the two
sides of one of these water barriers supposedly result from the
preservation in animals on one side, or on one island, of small
mutations, which would be swamped by crossbreeding if the water barrier
were not present. The effect of this isolation is easily seen if
ermines from the Queen Charlotte Islands are compared with those of the
opposite mainland. The degree of morphological difference is great.
Isolationwise, the Queen Charlotte Islands are the seaward end of a
chain, beginning with Admiralty Island in southeastern Alaska, and are
farther from the mainland, zoölogically, than the distance in actual
miles across the water channel would suggest. Between any two islands
that are geographically consecutive, however, and between the mainland
and the first island of the chain, the difference in the ermines is
small. In other places, water barriers of equal or greater width have
contributed little if anything to the differentiation from one another
of weasels on the two sides of the water barrier. The strait between
eastern Canada and Newfoundland is an example.
The absence of water, or scarcity of it to a degree that closely
approaches absence, in any large area appears to prevent weasels from
living there. At any rate, the one sizeable region of North America
from which weasels are unknown is the desert of the southwestern United
States and adjoining part of northwestern México. More precisely, in
western Arizona, the Mohave Desert and the desert of northwestern
Sonora, collectors of mammals have repeatedly sought small carnivores
without ever finding any weasels.
Degree of moisture is closely correlated with color in weasels.
Humidity and cloudiness as well as actual precipitation seem to be
involved. Even if we take into account average annual rainfall alone,
the darkest-colored weasels are found in the areas of heaviest rainfall
and the lightest-colored weasels in areas of lightest rainfall (extreme
type of desert where no weasels occur being excepted). In any large
region where there is a geographic gradient in rainfall, the transition
from light to dark color almost exactly parallels the increase in
amount of rainfall. Within a given species the same color reappears in
widely separated areas that have the same amount and seasonal
distribution of rainfall. This correlation is repeated so often that
one can almost certainly say that heavy rainfall, or the associated
phenomena of high humidity and cloudiness, acting separately or
together, causes an increase in intensity of color. Relative extent of
the color of the upper parts and underparts and presence and absence of
light facial markings seem also to be correlated, in a more general
way, with differences in rainfall. A fuller discussion of the nature
and amount of the variation in color is given on page 51.
Temperature seems not to be an important factor in directly limiting
the distribution of weasels, since _M. frenata_ occurs from the hottest
to some of the coldest parts of the Americas. Do _M. erminea_ and _M.
rixosa_ range no farther south, than they do at present, because high
temperatures constitute a barrier? No evidence is known to me which
provides an answer, one way or the other, to this question. Granting
that temperature is unimportant in limiting the distribution of
weasels, it seems to cause geographic variation. Increase in mean
annual temperature is correlated with decreased size in _M. erminea_
and with increased size in _M. rixosa_. Temperature, it seems, causes
the hair to vary; the pelage is harsher and sparser in weasels from
tropical regions than in those from boreal regions. Difference in
number of hairs is especially well shown on the soles of the feet. In
the weasels from the far north, the pads are concealed by hair and in
the weasels from the tropical regions the soles are mostly bare. Also,
the hair on the soles of the feet is longer in northern than in
southern weasels. Furthermore there is seasonal change in length of the
hair on the soles of the feet; at a given locality in southern Canada
the hair of the white winter coat is so long on the soles of the feet
as to obscure completely the palmar and plantar pads whereas the hair
of the brown summer coat is shorter and leaves these pads boldly
exposed to view. This seasonal change, as would be expected, is most
marked in animals of northern regions and is not perceptible in those
from the tropics; it is correlated with increase in seasonal change as
the distance from the equator increases.